The incidence of malignant glioma is approximately 12,000 new cases per year. These tumors represent the second leading cause of cancer mortality in people under the age of 35 and the fourth leading cause in those under the age of 54. The exact cause of the disease is unknown, although speculation exists regarding genetic predisposition, chemical, or viral causes. Whatever the cause, recent epidemiological evidence suggests a significant increase in the incidence of these tumors, particularly in the elderly.
The current approach to treatment usually represents a multi-modality approach. Depending upon grade and histopathology, surgery and/or radiation is utilized with or without cytotoxic chemotherapy. For the more advanced types of astrocytoma (Grades III–IV), combined modality approaches have had a questionable impact upon survival; median survival ranges from 40–50 weeks with most patients dead of disease at 2 years.
Despite recent advances in neuro-imaging, neuro-anesthesia, and neuro-surgical techniques, the prognosis of patients with malignant gliomas treated by surgical resection alone remains dismal with a median survival of 4–6 months. This reflects the unique infiltrative growth characteristics of malignant gliomas, which make true “total resection” impossible without causing unacceptable neurologic damage to the patient. To date, radiotherapy has proven to be the most effective treatment for malignant gliomas extending median survival to 8–9 months. Although adjuvant chemotherapy can prolong survival, few patients survive more than 18 months. Furthermore, once patients have tumor progression, conventional chemotherapy has not been shown to prolong survival. There are several reasons why gliomas are relatively resistant to standard chemotherapy including diminished drug delivery to the tumor secondary to the blood-brain barrier, tumor hypoxia, and their relatively low growth fraction. Most importantly, however, is the fact that gliomas tend to have significant intrinsic resistance to most standard cytotoxic agents. Research strategies have been aimed towards the development of new agents directed against novel cellular targets to be used either as single agents or in combination with currently available therapy of proven efficacy.
The development of more effective chemotherapeutic agents intended for combination with radiotherapy such that tumor cell kill is increased while maintaining or improving the therapeutic index has been the clinical rationale for the development of radiosensitizers. Desirable characteristics of clinically useful radiosensitizing agents would include a lack of systemic toxicity and selectivity towards the tumor cell population. Theoretically, the development of an effective radiosensitizer should be particularly appropriate in the management of malignant glioma as 90% of these patients will ultimately develop recurrences within a 2-cm margin of the original tumor, suggesting that these tumors are resistant to standard treatment doses of radiotherapy. In an effort to improve the survival of patients with malignant glioma, numerous clinical studies, both in the single institution and in the larger cooperative group setting evaluating potential radiosensitizers have been conducted. These agents have included halogenated pyrimidine analogs such bromodeoxyuridine and iododeoxyuridine, hypoxic cell sensitizers such as misonidazole and etanidazole, cytotoxic chemotherapeutic agents such as nitrosoureas, cisplatin, carboplatin and taxol, topoisomerase I inhibitors such as topotecan, inhibitors of protein kinase C such as tamoxifen and biological agents such β-interferon. In spite of this large clinical and laboratory effort to identify effective radiosensitizing agents, the overall survival of patients with malignant glioma has remained unchanged over the last two decades. In addition, one of the key drawbacks of most of these agents is their overwhelming systemic toxicity that often limits their clinical usefulness.
Perillyl alcohol (POH) is a monocyclic monoterpene. Monoterpenes are commonly and primarily produced by plants and are found in many commonly consumed fruits and vegetables, including citrus fruits and food flavoring such as mint. Monoterpenes occur in monocyclic, bicyclic, and acyclic forms and are either simple or modified hydrocarbons. We have demonstrated that sesquiterpenes have activities similar to the monoterpenes. We envision that they act with a similar mechanism.
The potential anticancer activity of limonene was first reported in 1971 by Homburger, et al. who observed that limonene, when co-administered with the carcinogen benzo-(rst)-pentaphene, resulted in inhibition of tumor development. These data were extended by Haag, et al. who demonstrated that limonene could cause regression of advanced rat mammary carcinomas.
Following in vitro screening, the naturally occurring hydroxylated monocyclic monoterpene POH was chosen for in vivo testing. Dietary POH was greater than five times more potent than limonene at inducing tumor regression. Dietary administration of POH caused 84% regression of rat mammary carcinoma induced by DMBA and 60% regression of rat mammary carcinoma induced by NMU. Limonene and POH are rapidly metabolized in the rat. Rats given a 2% POH diet for 10 weeks had plasma levels of terpene metabolites of 0.82 mM, while those fed a 10% limonene diet for 10 weeks had plasma levels of 0.27 mM. Thus, the difference in potency between limonene and POH may be due to differences in pharmacokinetics. The observed preclinical antitumor effect of perillyl alcohol has not been limited to mammary carcinoma. The laboratory of P. Crowell has observed an antitumor effect of POH in pancreatic carcinoma models. Only the POH-fed hamsters had either regression or no growth of the tumors while control animals showed tumor growth.
The exact mechanism of the antitumor activity of POH has not been established but several potentially important drug-related activities have been observed including: (1) G1 cell cycle arrest and induction of apoptosis; (2) Limonene and POH have been shown to inhibit isoprenylation of a class of 21–26 kD proteins, including small GTP-binding proteins involved in signal transduction, in a dose dependent manner at a point in the mevalonic acid pathway distal to 3-hydroxy-3-methylglutaryl coenzyme A reductase; and (3) Differential gene regulation including overexpression of the mannose-6-phosphate/insulin-like growth factor II (M6P/IGF II) and transforming growth factor-β (TGF-β) type II receptor genes.
Control and regressing POH-treated mammary carcinomas were examined by immunohistochemical methods and demonstrated increases in levels of both the M6P/IGF II receptor, as well as TGF-β, in treated regressing tumors compared with controls. Consistent with the potential importance of the M6P/IGF II receptor in POH-induced tumor regression, responding tumors had increased M6P/IGF II receptor levels compared to those of treated non-responding tumors. Liver tumors from POH-treated animals showed increased mRNA levels for the M6P/IGF II receptor and for the TGF-β type I, II, and III receptors compared with those of untreated animals. M6P/IGF-II also enhances the activation of TGF-β that acts as a mammary carcinoma mitogenic inhibitor and differentiating factor. In addition, POH inhibits the isoprenylation of small G proteins including ras-p21 that makes association of these proteins with the plasma membrane impossible and thereby inhibits cellular transformation.